The most fundamental tenet of building a startup is to understand a customer problem before building. Talk to users, find a problem, and build for that solution.
What are some unicorn startups founded in the last decade that didn't do that and found product-market fit?
Google, Facebook, Snapchat, TikTok, Reddit, Amazon, AWS, Apple/iPhone, Steam, Microsoft/Windows/Office, Revolut, Dropbox, lots of web hosting companies like DigitalOcean etc., lots of cryptocurrency companies like Cardano etc.
One notable exception that pivoted a lot is Twitter. It started as a podcast service iirc. Netflix is an interesting one too. They switched from DVDs to streaming which paid off for them.
Of course all have changed and added features as they have grown but they're all close to their original ideas which, imo, matches "build and they will come"